Lead author Professor Richard Duncan of the Institute for Applied Ecology at the University of Canberra says although the widespread extinction of birds in the Pacific between 700 and 3500 years ago has long been known, there has been debate over the true extent of the losses.

Estimates have ranged from about 800 to up to 2000 bird species and up to 8000 extinctions of local island populations.

Duncan says the main problem in understanding the loss of bird life has been the incomplete fossil record from the islands.

"Consequently, many extinct bird species remain to be discovered, confounding attempts to quantify more precisely the number and type of species lost across the region," write Duncan and colleagues Dr Alison Boyer from the University of Tennessee and Tim Blackburn, of London's Institute of Zoology.

Filling the gap

Duncan and colleagues have filled this gap by using the "mark-recapture approach" to estimate the number of unknown extinct species of nonpasserine - or non-perching - land birds.

These birds include species such as water fowl, birds of prey, parrots, ducks, geese and rails, as opposed to passerine or perching songbirds that include wrens, crows and lyrebirds.

"If you know what species are on an island now and can look to see how many of these are in the fossil record, you can estimate the probability of finding a species in the fossil record," he says.

"You can then use this to infer what you are missing."

The study looked at the fossil records of 41 remote islands in the eastern Pacific that were among the last to be colonised by humans.

A total of 618 populations of 193 nonpasserine landbirds were identified on the 41 islands, comprising 371 populations present at the time of European contact and 247 populations known only as fossils.

Duncan says this suggests nearly two-thirds of the landbird populations vanished in the years between the arrival of the first humans and European colonisation.

Mass extinction

The researchers extended these findings to the wider Pacific region - excluding New Zealand - and identified another 269 Pacific islands large enough and isolated enough to support island-endemic species.

"The results imply that human colonisation of remote Pacific islands caused the global extinction of at least 983 nonpasserine landbird species," they conclude.

Duncan says it is harder to put a figure on extinctions of nonpasserine sea birds as, because their range is widespread, it is harder to drive them to complete extinction.

And although it is thought that extinctions of passerine birds also occurred, this was more likely due to habitat loss as they were too small to be hunted.

"They suffered more after European arrival as it marked the arrival of predators such as rats and cats," says Duncan.

The study also shows wide variability in extinction rates between islands with lower rates of extinction on larger islands and on islands with higher rainfall.

Duncan says the conditions and topography of these islands meant there were areas of land either too mountainous or dense to be cut down.

"The persistence of forest habitat offered a refuge for birds," says Duncan.

Duncan says the study shows flightless species were 33 times more likely to go extinct than species able to fly due to overhunting as a food source by humans.

Species endemic to a single island were also 24 times more likely to go extinct that widespread species.

He says there is no doubt humans caused the mass extinction event: "This is pretty well accepted because it coincides so well with humans' arrival."